The invention relates to an applicator of microwave plasma generator applied to transmission of electromagnetic field into plasma and it further relates to a microwave plasma generator including the dealt with applicator.
A large scale of types of microwave plasma generators that can be, in general, characterized as closed generators where the plasma chamber is surrounded by metal walls forming a microwave resonator, or as open generators including among others microwave emitters, slow-wave structures and surfatrons, eventually generators with magnetic field based on electron resonance ECR (electron cyclotron resonance).
The known microwave plasma generator consists from a microwave power source (magnetron), wave-guide, circulator and from an applicator ensuring transfer of microwave energy into plasma filler (plasma bulk).
The Horn antenna is most frequently used in current microwave plasma generators as applicator, which in fact is a casing with an input in one side and with a rectangle profile cross section enlarging towards the output provided with a dielectric window on the other side. The applicator transmits linearly polarized waves. It is connected to a wave-guide in which also a circulator is connected necessary for conduction of reflected waves.
One of the general problems in transmission of electro-magnetic field to plasma is the fact that a certain part of the supplied power is reflected back. The generated plasma has some electrical conductivity and it therefore does not absorb all waves a part of which is then reflected back. This problem is significant especially in microwave generators since the reflected waves are transmitted via the wave-guide back to the source (magnetron). A long time charging of the magnetron by the reflected wave can result in the destruction thereof. To avoid this effect the reflected wave must be conducted outside of the magnetron. Therefore between the magnetron and the applicator into the track of the wave-guide a circulator is included to ensure the exhaust of the reflected wave and its transformation into heat. Even though the circulator is an efficient device, it is relatively complicated and expensive. It increases considerably the size, complexity and price of the known microwave plasma generators. Besides in order to tune the applicator, the microwave guide must be terminated by a movable short connection (movable wall), serving to tune the guide in order to achieve the maximum efficiency of the applicator.
The invention aims at creating such microwave plasma generator that could remove the above described deficiencies and that would allow an efficient elimination of a part of the reflected wave without the necessity of application of the above-described complicated, expensive and bulky elements.
This task is solved by formation of an applicator and a microwave plasma generator including the said applicator according to this invention.
The invention consists of the fact that it is possible to interfere the reflected waves by other reflected waves of the same wave length but with an appropriate phase shift. This is achieved by an applicator with a guiding part of the closed profile and with two outputs in the opposite ends. In the separations of both outputs and the plasma field reflected waves are generated and they reflect back against each other into the guiding part where they meet and interfere. An efficient elimination of the impact of the reflected waves on the microwave power source (magnetron) is surprisingly achieved by placing the magnetron directly in the input of the applicator placed in the wall of its guiding part in the distance A from the separation of the first output and in the distance B from the separation of the second output while the absolute value of the difference of the distances A-B or B-A equals λ/2 where λ is the wave length of the microwave from the microwave power source (magnetron). Distances A and B correspond to the trajectory of microwave propagation.
Then the waves from the opposite outputs meet in the guiding part in the place of the input of the microwave power source (magnetron) with exactly the opposite polarity, which disturbs their impact resulting in liberating the magnetron from the impact of reflected waves.
In one preferred embodiment of the applicator the guiding part is fixed from one side to the metal vacuum flange provided with apertures placed on the near side against outputs of the guiding part. On the reverse side, these apertures are covered by the dielectric cover.
It is also advantageous when the guiding part has the shape of a hollow body abutting the flange and covering the apertures by its outputs.
This embodiment has a very small size and great variability; it can be put in any device having an aperture corresponding to the circular flange.
In another preferred embodiment of the applicator according to the invention the guiding part has in the top view approximately a “U” shape the branches of which are provided by outputs in the ends and the input is placed in one of the branches. A longer track and greater stability characterize this embodiment.
A microwave plasma generator is also subject of the invention. It includes a microwave power source (magnetron) and an applicator where the microwave power source is connected to the guiding part of the applicator, provided on one of its ends by the first output, on the second end by the second output and the input is placed in the wall of the guiding part in the distance A from the separation of the first output and in the distance B from the separation of the second output while the absolute value of the difference of the distances A-B or B-A equals λ/2 where λ is the wave length of the microwave from the microwave power source (magnetron). Distances A and B correspond to the trajectory of microwave propagation.
The advantage of the above described applicator and microwave plasma generator including this present applicator consists in the fact that it is possible to exclude from construction of the device the expensive and bulky circulator including wave-guide and the tunable short connection of the microwave guide and that it is possible to create a simple, compact, easily transportable microwave plasma generator with a small size and low production costs.